Arty card holder

I wanted a more or less functional way of displaying things created with [Structure Synth](http://structuresynth.sourceforge.net/) and a card holder seemed like one option. Unfortunately Structure Synth does not import well into OpenSCAD so the base plate is added on in Structure Synth & needs to be re-positioned for different seed values. I had to print the 2 plates with 100% fill to avoid warping.